This invention relates generally to a microfilm reader and more particularly to a microfilm reading device for selectively positioning one of several projecting lenses relative to a micrograph, the projecting lenses being supported by a lens carrier which is movable relative to the microfilm reading device.
In the prior art microfilm readers projecting lenses are provided having different magnifications wherein the lenses are interchangeably mounted within a lens tube. Consequently, an operator can insert the lens having the desired magnification into the respective lens tube. In most cases, the use of two lenses with different magnifying factors is sufficient for a continuous operation. Therefore, microfilm readers are frequently offered as having two lenses mounted on a lens carrier. Accordingly the reader must have means for adjusting the respective lens relative to the micrograph.
In the prior art, there is known a microfilm reader which has an upper portion on whose bottom side a double lens carrier is slidably arranged above the micrograph to be projected. In order to position the respective lens relative to the micrograph during the sliding of the lens carrier, inclinations are provided in order to achieve that the respective lens is abutting a transparent glass plate usually covering the micrograph while the other lens which is not in use is lifted from the transparent plate. This principle seems to be sound, however, the structural solutions for carrying the principle into effect are rather complicated and expensive since inclinations are provided which must be constructed in a special manner.